The field of this invention is testing devices, particularly of the type used for testing the accuracy of an analyzer.
Prior art methods and apparatus for analyzer calibration and testing utilizing trace gases have taken the form of numerous, various types of systems. In one system, permeation tubes are used in such a fashion that permeation tube diffusion into a metered carrier gas flow is utilized for providing trace gases to an analyzer, such as that detailed in the article by Larry DeMaio of Mine Safety Appliances, Pittsburgh, Pa., entitled "Calibrating Trace Gas Analyzers Using Permeation Tubes", presented at the Eighteenth Annual Symposium of the Analysis Instrumentation Division of the Instrument Society of America, May 3-5, 1972 and published in Air Quality Instrumentation, Vol. 2, edited by J. W. Scales, Instrument Society of America, 1974, pages 229-241. Also, the article referenced Debbrecht, F. Jay and Neel, E. M, "Application and Description of a Portable Calibration System," Calibration in Air Monitoring, ASTM STP 598, American Society for Testing and Materials, 1976, pages 55-65, relates to systems based upon permeation tube techniques.
Other analyzer calibration techniques include dynamic dilution by means of a multi-stage process wherein the carrier gas and trace gas are measured by flow rate devices and mixed, with such techniques being detailed in the article Lucero, D. P., "Ultra Low-Level Calibration Gas Generation by Multi-Stage Dilution Techniques," Calibration in Air Monitoring, ASTM STP 598, American Society of Testing and Materials, 1976, pages 301-319. Still further, other systems utilize exponential dilution by diluting a gas in a fixed volume with a continous flowing carrier wherein the output concentration may be computed by formula.
Other techniques include passing a carrier gas over a liquid surface, with the carrier gas mixing with a pollutant gas in the liquid according to the partial pressure that the pollutant gas exerts over the liquid. Lastly, trace gases for analyzer calibration may be prepared by introducing a fixed volume of sample gas into a container of fixed volume of carrier gas to provide a given concentration of sample gas in the carrier gas.
However, with all prior art methods, typically low flow rates are difficult to accurately measure; or in the alternative, the sample gas-carrier gas mixtures, after preparation, tend to deteriorate with time due to absorption reactions, adsorption reactions, and permeatation, which tend to provide results which are unreliable for accurately calibrating an analyzer. Some prior art systems are dependent on flowing very small quantities of a sample gas in a carrier gas stream where measurements are not easily monitored, particularly in current, state of the art flow meters. Further, many of the permeation devices are subject to deviation if the temperature and barometric pressure are not held within a critical range. Fluctuations in temperature and the barometric pressure affect permeation rates which can and do affect the reliability of permeation tube devices.
Prior art devices include those such as disclosed in U.S. Pat. Nos. 2,927,465; 3,253,469; 3,362,228; 3,412,935; 3,479,880; 3,533,295; 3,535,939; 3,654,959; 3,681,996; 3,746,217; 3,751,992; 3,908,463; 3,915,013; and, 3,975,946. Of these references, it is useful to note that microliter valves in the past have been disclosed and used in the use of a chromatograph apparatus, which is well known in the art. However, the use of a microliter valve in a device for testing and calibrating the accuracy of an analyzer is believed to be heretofore unknown.